Synaptic development and function require assembly of protein complexes at synaptic sites. A key breakthrough in understanding molecular mechanisms for synapse development was the discovery that PDZ proteins play central roles in scaffolding receptors and signaling elements at pre- and postsynaptic sites. Mutations in lin-2, lin-7 or lin-10, genes encoding PDZ proteins in C. elegans, disrupt vulval differentiation. Elegant biochemical and genetic analyses indicate that LIN-2/-7/-10 form a protein complex that mediates subcellular localization of an EGF receptor essential for vulval development in worms. The LIN-2/-7/-10 complex also occurs prominently in neurons. Disruption of lin-10 in C. elegans prevents proper postsynaptic sorting of a glutamate receptor. In mammals, close homologues of LIN-2 (CASK), LIN-7 (MALS/Veli) and LIN-10 (Mint/X11) have all been identified and the LIN-2/-7/-10 complex occurs at highest levels at pre- and postsynaptic junctions in mammalian brain. We found that the PDZ domains of MALS bind the C-termini of NMDA receptor subunits, and biochemical studies implicate the CASK/MALS/Mint complex in microtubule-dependent trafficking of NMDA receptors. Based on these data, we hypothesize that MALS regulate receptor trafficking at mammalian cell junctions, especially synapses. To address this hypothesis, we are generating mutant mice that lack the three MALS isoforms (MALS-1/- 2/-3). We previously reported that mice lacking MALS-1 and MALS-2 are without any discernable phenotype. In preliminary unpublished studies, we now find that MALS-3 knockouts (KOs) have developmental defects. Whereas mice that lack any pair of MALS genes are viable for at least one week of life, triple mutant mice die within 1 hour of birth. Furthermore, hypomorphic mutants that lack MALS-1/-3 or MALS-2/-3 have prominent neurological phenotypes. We now plan to study these mutant mice to determine the essential roles for MALS in regulating ion channels and growth factor receptors at mammalian cell junctions, especially synapses. Because MALS proteins regulate growth factor receptors and interact with NMDA type glutamate receptors at synapses, this work will be relevant to both tissue development and synaptic plasticity.